The invention relates generally to the field of gaskets for shielding electromagnetic interference (EMI). In particular, the invention concerns gaskets which include EMI shielding members supported by resilient structures such as, in some embodiments, a multiplicity of discrete, elongate filaments which may or may not be electrically conductive.
Most electronic equipment generates unwanted electromagnetic energy during operation which, if not properly shielded, can interfere with other electronic equipment by way of radiation and/or conduction. This can include electromagnetic energy of wavelengths along various points of the spectrum such as radio frequency interference. As used herein, therefore, the term electromagnetic interference pertains to interfering electromagnetic energy of any wavelength.
To avoid the problems associated with electromagnetic interference, all sources of electromagnetic energy should be properly shielded and grounded. Shielding should be designed to prevent both incoming and outgoing interference. In the case of equipment permanently contained in a housing, effective shielding can be accomplished through proper construction of the housing. It is difficult to shield effectively, however, equipment having access panels or doors. This is because the seams between the access panels or doors and the housing body provide natural escape and entry routes for electromagnetic interference. In the case of metal housings, gaps can also inhibit the beneficial Faraday Cage effect by presenting conductivity discontinuities in the electrically conductive surfaces of the housings. The gaps also reduce the efficiency of the ground conduction path. Moreover, by presenting an electrical conductivity level which is significantly different from that of the housing, the gaps can act as slot antennae and result in the housing becoming a secondary source of EMI radiation.
In the case of electrical apparatus housings which include seams, therefore, an effective shielding mechanism must be configured both to provide sufficient EMI shielding characteristics and to allow proper closure of the door or access panel. Various configurations of gaskets have been developed for serving these purposes. They are intended to provide maximum EMI shielding, while minimizing the force required to close the door or access panel.
To present an effective shield to electromagnetic interference, a gasket should be capable of absorbing and/or reflecting electromagnetic interference and of establishing as continuous an electrically conductive path as possible across the seam in which the gasket is positioned. Typically, metallic structures are used for EMI shielding because of their high degree of electrical conductivity. Because that conductivity is not infinite, however, part of the electromagnetic field being shielded is transmitted across the shield and supports a current in the shield. Accordingly, an effective EMI shielding gasket should provide an electrically conductive path for dissipating this current to ground, otherwise the current could serve to radiate an electromagnetic field on the other side of the shield.
Known EMI shielding gaskets have been unable to combine effectively these qualities with the other requirements of suitable gaskets, such as presenting a consistent, minimal closure resistance. Another important characteristic of an EMI shielding gasket is that it not break down due to galvanic corrosion, such as can occur when dissimilar metals are in contact with one another.
It is an object of the invention, therefore, to provide a gasket which affords improved shielding of electromagnetic interference passing through seams in electronic apparatus housings. Another object of the invention is to provide such a gasket which is of a simple, cost effective construction. Still another object of the invention is to provide such a gasket which presents a narrow profile and thereby presents a minimum of closure resistance to housing doors and access panels.